This invention is generally concerned with magnetic distortion reduction in loudspeaker systems. U.S. Pat. No. 5,070,530, issued Dec. 3, 1991 , to the inventors describes and claims a technique for reducing magnetic distortion that involves correcting for the effects of signal-related AC fields produced by the moving voice coil of the loudspeaker magnetic structure. The distortion producing effects described are inherent in the design of prior art loudspeaker magnetic structures and act both directly on the permanent magnet in the form of AC magnetic fields and indirectly through the magnetic and electric fields resulting from eddy currents which are produced by the voice coil energy. The patent teaches loudspeaker distortion reduction by incorporating one or more radial slots in one or more elements of the magnetic structure, i.e. the permanent magnet, and the top and bottom plates. The slots, which act as both electrical and magnetic barriers, effectively reduce circulating eddy currents in the plates, and magnetic and dielectric currents in the magnet.
The present invention is predicated on the fact that the pole piece contains a high concentration of AC energy, both as magnetic flux and as eddy currents because of its unique location within the voice coil. The invention, which involves isolation and redirection of the AC fields and the eddy currents associated with the pole piece to minimize interaction with the magnet and the bottom plate, has been found to substantially reduce loudspeaker distortion. In accordance with one aspect of the invention, the pole piece is transversely divided into two (or more) electrically isolated sections which interrupts the electric connection between the pole piece and the bottom plate, thereby decreasing the electric conduction of pole piece eddy currents in the bottom plate. The high concentration of AC flux in the pole tip (which has been isolated from the bottom plate) is routed into new magnetic return paths to minimize interaction with the magnet. The new magnetic return paths can be arranged around a single speaker driver in a single driver loudspeaker system or they may connect between speaker drivers in multiple driver loudspeaker systems. In multiple driver loudspeaker systems, the invention also provides the means for establishing mutual magnetic AC feedback between the drivers, which AC feedback further lowers distortion and minimizes the effects of differences between individual drivers. The effect is to minimize inaccuracies flowing from the lack of identical output from the individual drivers in multiple driver loudspeaker systems. The invention is also of benefit in loudspeaker systems that utilize crossovers for directing signals in various frequency bands among different drivers. Integration of the different size drivers to effect a coherent, seamless transition through the crossover region is greatly improved with the invention. Thus the lower distortion and more uniform driver characteristics that result from the invention produce substantial improvements not only in individual speaker drivers but in multiple driver loudspeaker systems.